Refractory composition



United States Patent O 3,294,562 REFRACTORY COMPOSITION Mario J. Caprio and Rudolf F. Krisleit, New Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N Drawing. Filed Dec. 11, 1964, Ser. No. 417,807 Claims. (Cl. 106-64) This invention relates to refractory compositions and to a method for their preparation. More particularly, this invention relates to a new and improved heat insulating refractory which can be used in contact with molten, nonferrous metals having melting points below about 1000 C., e.g. molten aluminum and aluminum base alloys.

Numerous insulating refractories now commercially available exhibit adequately low thermal conductivity. In addition, several :of these have acceptable resistance to metal attack, i.e. chemical reaction or wetting. These materials are, however, lacking one or more of the properties which an acceptable refractory should possess, i.e. dimensional stability, spall resistance, good machinability, and resistance to thermal and mechanical shock.

Refractory compositions employing an adhesive, such as calcium aluminate cement, as a bonding agent in combination with refractory aggregates such as diatomaceous earth, silica, magnesia, clay, alumina, expanded shale, crushed firebrick, mineral wool, fiber glass, asbestos and the like or combinations of such aggregates, in various proportions and formulations have been known. Nevertheless, such refractory compositions have not possessed the requisite properties enumerated above.

It has now been discovered that an insulating refractory possessing the desired properties may be produced from a composition which includes fibrous materials containing silica-alumina, or potassium titanate; an asbestos fiber, and a calcium aluminate binder.

It is therefore an object of this invention to provide a heat insulating refractory composition exhibiting characteristics of low thermal conductivity, resistance to Wetting and dimensional stability.

It is a further object of the invention to provide a heat insulating refractory composition which will resist thermal and mechanical shock and resist spalling.

Yet another object of the invention is to provide a heat insulating refractory composition which exhibits good machinability.

These and other objects will be more fully understood and appreciated from the following detailed description which is exemplary and explanatory of the invention, but not restrictive thereof.

The refractory of the present invention, in its broader aspect, comprises a dry mixture of certain fibrous mate rials and a hydraulic setting binder or cement adapted to be mixed with water to form a slurry which is subsequently pressed into a predetermined form which may have a slab or board-like configuration and cured at elevated temperatures to establish a ceramically bonded heat insulating refractory. The hinder or bonding agent is preferably a calcium alum-inate cement such as Alcoa CA-25 cement. An asbestos fiber is employed along with other suitable fibrous materials such as an aluminasilica fiber or potassium titanate fiber. Among the commercially available alumina-silica fibers are the materials sold under the trade names of Fiberfrax and Kaowool. Tipersul is a trade name for a commercially available potassium titanate fiber. In addition, known adhesive and filler materials may be added to the composition.

The general formulation of the invention comprises 40 to 60% by weight of asbestos fiber, 30 to by weight of calcium alumin-ate cement, and 30 to 20% by weight of a fibrous material selected from the group consisting of fibrous alumina-silica which is refractory and substantially inert toward molten aluminum and fibrous potassium titanate; The alumina-silica fibrous material preferably consists of about equal parts of both fibrous components. It is, however, only necessary that the proportions of the two components are such that the resultant alumina-silica fibrous material is refractory and substantially inert toward molten aluminum. For example, an alumina-silica fibrous material having 99% silica, such as the material sold under the trade name Refrasil may he used. Alternatively, fibrous potassium titanate may be used in place of the alumina-silica fibrous material. It has been found that the addition of more than about 60% by weight of asbestos fiber results in a refractory which lacks strength. On the other hand, if the alumina-silica or potassium titanate' component exceeds about 30% by Weight, the resultant refractory exhibits undesirable brittleness.

In some applications it'may be desirable to increase the flexibility of an alumina-silica fibrous material having a very high silica content. This may be conveniently accomplished by the addition of a relatively small quantity of any of the well known bonding agents.

Refractories made from these formulations have been found to have a low thermal conductivity of about 2-4 B.t.u./hr./ft. F./in. The machinability has been found to be superior to known refractories with fewer ragged portions and broken fibers appearing on machined edges. The improved dimensional stability which our refractory exhibits has resulted in no appreciable shrinkage and an absence of cracking and spalling. These refractories have also exhibited good resistance to thermal and mechanical shock as well as resistance to wetting.

While improved properties result from any formulation falling within the ranges stated above, the preferred range comprises 45 to 55% by weight of asbestos fiber, 28 to 22% by weight of calcium aluminate cement, and 28 to 22% by weight of either fibrous alumina-silica or fibrous potassium titanate. The preferred composition comprises 50% by weight of asbestos fiber, 25% by weight of alumina-silica fibrous material or fibrous potassium titanate, and 25% by weight of calcium .aluminate cement.

In preparing the refractory board, approximately 300% water, by weight of total solids, is added to the dry composition. Mixing may be accomplished by mechanical means such as a Hobart mixer, if desired. The resulting slurry is pressed into a slab or board-like form at about 1000 psi. and is fired at temperatures in excess of 1400 F. and preferably from about 1400 F. to 2000 F. with a maximum temperature rise of 100 F. per hour. The preferred curing period should be at least about 2 hours for each inch of thickness of the slab and preferably about 6 to 8 hours. The curing process results in driving oif both mixing water and water of crystallization, and the formation of a ceramic bond. Optionally, the slab may be autoclaved prior to curing.

Whereas particular embodiments of the invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in be appended claims.

We claim:

1. A ceramically bonded fibrous heat insulating refractory composition consisting essentially of 40 to 60% by weight of asbestos fiber; 30 to 20% by weight of calcium al-uminate cement; 30 to 20% by weight of a fibrous material selected from the group consisting of fibrous alumina-silica which is refractory and substantially inert toward molten aluminum and fibrous potassium titanate,

having a thermal conductivity of about 2-4 B.t.u./hr./ ftF/ F./in.

2. A ceramically bonded fibrous heat insulating refractory composition consisting essentially of to by weight of asbestos fiber; 28 to 22% by weight of calcium aluminate cement; 28 to 22% by weight of a fibrous material selected from the group consisting of fibrous alumina-silica which is refractory and substantially inert toward molten aluminum and fibrous potassium titanate, having a thermal conductivity of about 2-4 B.t.ui/ hr./ ft. F./in.

3. A method of producing a cera-rnically bonded heat insulating refractory slab of a fibrous refractory composition consisting essentially of 40 to by weight of asbestos fiber; 30 to 20% by weight of calcium aluminate cement; 30 to 20% by weight of a fibrous material selected from the group consisting of fibrous alumina-silica which is refractory and substantially inert tow-ard molten aluminum and fibrous potassium titanate; consisting essentially of mixing said composition with about 300% water by weight of total solids; compressing the resulting slurry into slab form at a pressure of about 1000 pounds per square inch and firing said slab at a temperature in excess of 1400 F. for a period of at least two hours for every inch of slab thickness with a temperature rise of not more than F. .per hour.

4. The method of claim 3 wherein the firing temperature does not exceed 2000 F.

5. The method of claim 3 wherein the slab is autoclaved prior to firing.

References Cited by the Examiner UNITED STATES PATENTS 2,509,599 5 1950 Hollenberg 106-99 2,793,128 5/ 1957 Emhiser 10664 References Cited by the Applicant UNITED STATES PATENTS 2,516,892 8/1950 Lobaugh. 2,793,128 5/ 7 Emhiser. 3,014,872 12/1961 Scott. 3,017,318 1/1962 Labino. 3,079,267 2/ 1963 Konrad.

TOBIAS E. LEVOW, Primary Examiner.

J. POER, Assistant Examiner. 

1. A CERAMICALLY BONDED FIBROUS HEAT INSULATING REFRACTROY COMPOSITION CONSISTING ESSENTIALLY OF 40 TO 60% BY WEIGHT OF ABESTOS FIBER; 30 TO 20% BY WEIGHT OF CALCIUM ALUMINATE CEMENT; 30 TO 20% BY WEIGHT OF A FIBROUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF FIBROUS ALUMINA-SILCA WHICH IS REFRACTORY AND SUBSTANTIALLY INERT TOWARD MOLTEN ALUMINUM AND FIBROUS POTASSOIM TITANATE, HAVING A THERMAL CONDUCTIVITY OF ABOUT 2-4 B.T.U./HR./ FT.2/*F./IN. 